Subliminal instructional messages are aural, symbolic, or textual messages presented with an often unrelated visual, aural, or audiovisual presentation (hereinafter the underlying video). These messages are presented in such a manner as to not be distracting to the viewer of the underlying video, which is frequently an entertainment oriented presentation. Subliminal messages are intended to be recognized by the viewer's subconscious mind, where they may eventually lead to behavioral modification.
Subliminal messages may be incorporated into a video signal for viewing on a television receiver. This may be done by substituting a frame or field of the video signal with the message, while the majority of fields or frames are those of the underlying video. Subliminal messages may also be presented by weakly modulating a visual characteristic, such as brightness, of the underlying video.
When a subliminal message is presented by weakly modulating a visual characteristic of the underlying video signal, it is important that superimposed subliminal messages be synchronized to the underlying video signal. If the superimposed message's frame and line rates differ greatly from those of the underlying video, the superimposed message may be become so broken up as to be illegible. Should the rates be closer, the message may wander, or roll, about the screen in such a way as to be distracting to the viewer.
Similarly audio subliminal messages can be superimposed upon an audio signal by mixing them with the underlying audio signal. It is, however, advantageous to increase the amplitude of the subliminal audio signal during those periods when the underlying audio signal is loud relative to the amplitude of the subliminal audio signal during those periods when the underlying audio signal is soft.